The intervertebral disc is an anatomically and functionally complex joint. The intervertebral disc is composed of three component structures: (1) the nucleus pulposus; (2) the annulus fibrosus; and (3) the vertebral end plates. The biomedical composition and anatomical arrangements within these component structures are related to the biomechanical function of the disc.
The spinal disc may be displaced or damaged due to trauma or a disease process. If displacement or damage occurs, the nucleus pulposus may herniate and protrude into the vertebral canal or intervertebral foramen. Such deformation is known as herniated or slipped disc. A herniated or slipped disc may press upon the spinal nerve that exits the vertebral canal through the partially obstructed foramen, causing pain or paralysis in the area of its distribution.
To alleviate this condition, it may be necessary to remove the involved disc surgically and fuse the two adjacent vertebrae. In this procedure, a spacer is inserted in the place originally occupied by the disc and the spacer is secured between the neighboring vertebrae by the screws and plates or rods attached to the vertebrae. Despite the excellent short-term results of such a “spinal fusion” for traumatic and degenerative spinal disorders, long-term studies have shown that alteration of the biomechanical environment leads to degenerative changes particularly at adjacent mobile segments. The adjacent discs have increased motion and stress due to the increased stiffness of the fused segment. In the long term, this change in the mechanics of the motion of the spine causes these adjacent discs to degenerate.
Artificial intervertebral replacement discs may be used as an alternative to spinal fusion.